Wave energy radiator



K. ,C. KELLY WAVE ENERGY RADIATOR Oct. 6, 1959 Fil ed Jul 1, 1957 2 Sheets-Sheet 1 FIG.

KENNETH c. KELLY,

INVENTOR BY y.

AGENT Filed July 1,. 1957 K. C. KELLY WAVE ENERGY RADIATOR 2 Sheets-Sheet 2 FIG. 2.

FIG. 3.

KENNETH C. KELLY,

INVEN TOR www- AGENT United States Patent WAVE ENERGY RADIATOR Kenneth C. Kelly, Los Angeles, Calif., assignor to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application July 1, 1957, Serial No. 669,913

13 Claims. (Cl. 343-771) This invention relates to wave energy radiators and more particularly to an antenna for radiating a linearly polarized pencil beam.

Electromagnetic energy beams of circular cross section which are commonly referred to as linearly polarized pencil beams are extensively utilized for the location or tracking of reflecting objects, and for point to point microwave communication lines. Heretofore linearly polarized pencil beams have been obtained, for example, by a combination of a linearly polarized electromagnetic wave source and a reflector. Such reflectors often occupy a large volume, however, and therefore have not been found too desirable in systems where space was limited.

Another method which has been employed for the production of linearly polarized pencil beams utilized spatial arrays of linearly polarized sources such as a plurality of dipoles or slots. Such systems have sometimes had the advantage of elimination of the space wasting reflector. However, these arrays are usually diflicult to fabricate and further often require a complicated feed system imposing complex electrical requirements. Radiating horns have also been utilized for the production of linearly polarized pencil beams. While such horns have avoided the complicated feed system attendant in spatial arrays, they have in many cases required a great depth in the direction of the radiating beam and have been found unsuitable when space was at a premium. The same limitations are usually present in surface wave antennas which are also capable of producing linearly polarized pencil beams. As is well known, a surface wave antenna suitable for the production of a linearly polarized pencil beam requires very great length to sutficiently narrow the beam in a plane perpendicular to the surface.

Recent attempts to develop a radiator to produce a linearly polarized beam have utilized annular slots excited by a structure setting up either radial or circumferential electric fields in the slots but not both. The beams so produced are, however, elliptical in cross section and the side lobe level varies with the plane of ob servation. Furthermore, there are directions in space in which prominent cross polarization exists.

It is therefore an object of this invention to provide an antenna for radiating a linearly polarized pencil beam which is excited by a simple feed structure.

It is also an object of this invention to provide a linearly polarized pencil beam radiator which occupies a small volume and has a small depth.

It is a still further object of this invention to provide a linearly polarized pencil beam antenna which is simple to manufacture and which has a low side lobe level.

It is still a further object of this invention to provide an improved linearly polarized pencil beam radiator which is simple in construction, reliable in operation, small in volume, light in weight and easily controllable.

In accordance with one embodiment of the pencil beam antenna of this invention, a conductive plate is perforated with a large number of radiation apertures disposed along a number of aperture circles. Currents are induced into 2,908,001 Patented Oct. 6, 1959 the plate by electromagnetic Waves in such manner that the gap produced by any aperture intercepts a portion of the current and causes excitation of that aperture. The directions of the currents induced into the plate and the locations of the apertures in the plate are selected in such a manner that the electromagnetic waves radiated into space by each aperture are all linearly polarized along a common plane of polarization.

This result of linear polarization along a common plane is accomplished in the selected embodiment by inducing a set of circumferential bands of radial and circumferential current into the surface of the conductive plate. The centers of the radial and the circumferential currents are separated by such a distance that a series of concentric circles may be found on which the vector sum of the currents due to the circumferential bands of radial and circumferential currents are parallel to one another and thereby determine the common plane of polarization. These circles, which are referred to as the concentric aperture circles, determine the location of the radiation apertures. The distance of separation of the centers must also meet the condition that the vector sum of the currents are substantially equal in magnitude over any one of the series of concentric aperture circles. Therefore, the radiation from the radiation apertures will provide a linearly polarized beam of circular cross section having a plane of polarization parallel to the vector sum of the exciting currents.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which an embodiment of this invention is illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, vand are.

not intended as a definition of the limits of the invention.

Fig. 1 is an exploded perspective view, partially cut away, of one embodiment of the radiator of this invention;

Fig. 2 is a vector diagrammatic view of the circumferential and radial current bands in the conductive plate of the radiator of Fig. l; and

Fig. 3 is an illustrative diagram of the geometry of the radiator of Fig. 1 showing the relative displacement of the center of the aperture circle, the feedpoint of the circumferential currents and the feedpoint of the radial currents.

Referring now to the drawings, in which like reference characters designate like parts, there is shown in Fig. 1 a planar conductive plate 10 perforated with a large number of radiation apertures 12. The radiation apertures 12 are uniformly disposed in the plate 10 on three annular concentric aperture circles 14, 16 and 18. The circles 14, 16 and 18 have a common center at a point, indicated in Fig. l by reference character 20, and have a substantially circular cross section. As will be indicated below,

the radiation apertures 12 may be any cross-sectional shape such as oval, elliptical, cross-shaped, square, or

The circular cross section is merely a substantially larger number of aperture circles may be found preferable in practice.

The conductive plate 10 is to be excited by standing wave field modes which induce circumferential bands of.

radial and circumferential currents therein. The term circumferential band of current is defined as a surface current whose magnitude and direction is constant over a given circle. In other words the magnitude and direction is only a function of the radial direction from a center. The insertion of the term radial or circumferential before current indicates the direction of current flow. A radial current is a current flowing in a radial direction and a circumferential current is a current flowing in a circular direction. The wavelength of the induced circumferential current is substantially equal to the wavelength of the induced radial current, this wavelength being referred to as the working wavelength. The radii 22, 24 and 26 of the aperture circles 14, 16 and 18 respectively differ from one another by an integral number of working wavelengths. In other words, if the radius 26 is taken as unity, the length of radius 24 is unity plus pk where p is any integer and A is one working wavelength.

The conductive plate is coupled to a radial waveguide 30 having a bottom plate 32. The radial waveguide 30 is terminated by a cylindrical conductive wall 34 which is concentric with an axis 36. The axis 36 may be termed the axis of symmetry of the radial waveguide or the first axis 36. An axis 38 may be defined which passes through the center 20 of the aperture circles and extends parallel to the axis 36. The axis 38 may be termed the axis of symmetry of the radiation apertures or the third axis 38. The distance between first axis 36 and the third axis 38 is equal to one-quarter of the working wavelength.

The bottom plate 32 includes a circular feed aperture 40 concentric with the first axis 36. The circular feed aperture 40 provides access to a feed means 42 for exciting the radial waveguide 30 so that circumferential and radial currents may be set up in the plate 10. The feed means 42 includes a circular waveguide 44 having a spline filter or webbing 46. A coaxial waveguide 48 with in the cylindrical waveguide 44 so that its outer conductor 50 is flush with the top of the spline filter 46 and its inner conductor 52 projects into the radial waveguide a short distance. The inner conductor 52 is coaxial with a second axis 54 parallel to third axis 38 and which penetrates the conductive plate 10 at the point 56.

The distance between the third axis 38 and the second axis 54 is one-quarter of a working wavelength. The three axes 36, '38 and 54 may be said to define the vertices of an isosceles triangle wherein the axis 38 passes through the vertex of a 90 angle of the triangle and the axes 36 and 54 each pierce the vertex of 45 angles of the triangle. Consequently, the distance between the second axis 54 and first axis 36 is equal to A plurality of thin conductive ribs or fins 58 are disposed within the radial waveguide 30 to provide an electrical termination to a selected radial waveguide mode. The outer end of the conductive fins 58 terminates at the cylindrical wall 34. Even though the fins themselves extend radially toward the axis 36, the inner end of the fins terminate to define a cylindrical wall 60 which is concentric with the second axis 54. The fins 58 are only a preferred embodiment of a short circuit member and may be replaced by other selectively shorting structures which define a cylindrical shorting wall 60. The important feature of the thin conductive fins is that they act as a short circuit to selected electromagnetic wave modes which short circuit must be concentric with the second axis 54 while leaving other modes undisturbed to progress to the short circuit at the cylindrical wall 34.

The operation of the radiator will now be explained in conjunction with the aid of Fig. 2 and Fig. 3. The circular waveguide 44 is dimensioned for propagation of the TE -mode of the circular waveguide. When the TE -mode of the circular waveguide is coupled into the radial waveguide 30 by the circular waveguide 44 the H -mode of the radial waveguide is excited between the conductive plate 10 and the bottom plate 32. The cylindrical wall 34 provides a short circuit to the H -mode of the radial waveguide and serves the function of setting up standing waves. The working wavelength is here the asymptotic value of the governing Bessel function. These standing waves induce standing wave circumferential bands of circumferential current. The centers of the bands, 70, carry the most intense current of the bands in the inner surface of the conductive plate 10 as shown in Fig. 2. The distance between neighboring band centers 70 is approximately equal to one-half of a working wavelength, the exact distance being determined by the appropriate Bessel function. The fins 58 permit the unhindered propagation of the H -mode since the electric vector is at all times perpendicular to the plane of the fin and since the fins are vanishingly thin. It is for this reason that the fins are radial with respect to axis 36, the center of the curernt bands around 70. The broken lines of Fig. 2 show the circular current band centers 70 set up in the conductive plate 10. The distance between the circumrferential current band centers 70 and the circular wall 34 which acts as a short circuit to the H -mode is approximately an odd number of one-quarter working wavelengths as is well known to those skilled in the art. More particularly, the lines 70 are found by knowing the zeroes of the applicable Bessel function.

The coaxial waveguide 48 is dimensioned for propagation of the TEM-mode of the coaxial waveguide and when excited will set up the E -mode of the radial waveguide between the conductive plate 10 and the bottom plate 32. The E -mode produces the desired current bands with radially directed currents in the conductive plate 10 because the cylindrical wall 60 concentric with the axis 54 provides a short circuit for the E -mode which in turn sets up standing E -mode waves. Fig. 2 by way of illustration, shows the centers of these circumferential bands of radial currents by the lines designated by the reference character 72. The current is most intense in the center of the circumferential bands of radial currents. These bands are concentric with the axis 54. The spline filter 46 located within the circular waveguide 44 supplies a conductive surface for the E -mode which induces the radial current.

Both the circumferential bands of circumferential current 70 and of radial current 72 are periodic time varying in amplitude as is implicit in the term standing wave. Thus Fig. 2 gives an instantaneous picture.

As is well known to those skilled in the art the height of a radial waveguide determines the modes propagated therein. Even though the operation of the invention has been explained in terms of the H -mode and the E -mode of the radial waveguide, several other mode pairs will likewise produce the desired current bands in the conductive plate 10. Generally speaking, the H -mode and E -mode of the radial waveguide may be utilized for the production of the bands of radial and circumferential currents, n being an integer.

The operation of the combination of the feed means 42 and the radial waveguide 30 may be summed up thus. A standing wave of the H -mode of the radial waveguide is excited within the radial waveguide symmetric about the first axis 36. This H -mode produces circumferential bands of circumferential currents in the conductive plate 10. Also a standing wave of the B -mode of the radial waveguide is excited within the radial waveguide symmetric about the second axis 54. This E -mode produces circumferential bands of radial currents in the conductive plate 10. Since the axes of symmetry of the circular and radial current bands are displaced from one another, the excitation of an aperture in the conductive plate 10 depends on the location of the aperture.

It may therefore be seen that when apertures are placed upon an aperture circle, such as aperture circle 74 (shown in Fig. 2) having its center on the third axis 38, such a cirple is very nearly the locus of all points for which the resultant of the vector sum of the circumferential and radial electric currents are equal in magnitude and direction. If the modes excited within the waveguide are the H and E -modes, the three axes 36, 38 and 56 still have to define the vertices of the isosceles triangle where equal sides are one-quarter of a working wavelength long.

Referring more particularly to Fig. 2, there is shown an aperture circle 74 on which selected radiation apertures are disposed. The circles 80, 82, 84 and 86 represent these selected radiation apertures which are pulled out of the drawing as indicated by the arrows for the purpose of further explaining the operation of this invention. These explanations apply for the instant when the currents are as shown by 70 and 72.

Referring nowto aperture 80, it can be seen from the drawing that the center of the band of circumferential currents intersects the aperture circle 74 so that the circumferential current is a maximum. The electric field component excited by the circumferential current band is designated as C. The aperture circle 74 is also located on a path midway between bands in which currents flowing towards and currents flowing away from the axis 56 exist. Consequently, the bands of radial current will not excite the radiation aperture 80 so that the sole excitation is due to the circumferential current component C.

Referring now to aperture 82, the situation is just the reverse. The radiation aperture 82 is located midway between two bands of circumferential currents which position causes no excitation thereof. The sole excitation is due to the radial current band which produces a component R in the same direction and of the same magnitude as the component C from radiation aperture 80.

Radiation apertures 84 and 86 respectively obtain contributions from both the circumferential currents and the radial currents. These electric currents excite respectively the components C and R as indicated. When these components are vectorially added the resultant 90 and 92 are obtained providing an electric field in these holes of the same magnitude and in the same direction as the fields in radiation apertures 80 and 82. Since these fields are time varying, the resultant electromagnetic radiation will be linearly polarized.

Fig. 2 shows with greater particularity the geometry of the radial waveguide of Fig. 1. The cylindrical wall 34 is concentric with the first axis 36 and provides the short circuit to the H -mode of the radial waveguide. The cylindrical wall 60 defined by the inner ends of the fins 58 is concentric with the second axis 54 and provides a short circuit to the E -mode of the radial waveguide. The aperture circle 74 is concentric with the third axis 38 and provides the locus of all the points where the vector sums of the currents due to the radial and circumferential currents are parallel to one another and equal in magnitude. The arrows drawn from the axis to the three circles 34, 60 and 74 are merely included to indicate the respective center of these circles. The arrows on 70 and 72 indicate the directions of the currents at a given instant.

The impedance presented to the radial waveguide by the radiation apertures is easily controllable through aperture diameter, aperture spacing on the aperture circle, radius of the aperture circle and the number of aperture circles. Also, the apertures may be cut, punched or etched in a conductive plate with great speed and simplicity in contradistinction to the time and effort which are expended in cutting slots into waveguide walls For even greater simplicity of manufacture and extremely light weight, the radial waveguide may be constituted of metal foil clad dielectric plates separated by a honey combed plastic. The apertures could be etched into the metal clad dielectric by photo chemical methods.

There has been described a radiator which may be utilized for radiating a linearly polarized pencil beam which essentially comprises a conductive plate perforated with radiation apertures. Means are provided for exciting within the conductive plate the circumferential bands of radial and of circumferential current whose respective centers of symmetry are separated by a quarter of a working wavelength. The radiation apertures are disposed within the conductive plate in such a manner that the magnitude of the resultant of the vector sum of radial and the circumferential currents are equal in magnitude and parallel in direction. The radial waveguide, the feed means and the short circuit utilized for setting up standing waves illustrates but one method of obtaining the desired currents; other methods for setting up such a current configuration will suggest themselves to those skilled in the art.

What is claimed as new is:

l. A radiator comprising: a planar conductive plate including radiation apertures; a radial waveguide including a planar wall, and being coupled to said conductive plate; and means coupled to the planar wall of said waveguide for exciting two different modes within said radial waveguide which excite the radiation apertures with energy forming a linearly polarized beam.

2. A radiator comprising: a planar conductive plate including radiation apertures disposed along a plurality of concentric circles; a radial waveguide having a planar wall and a cylindrical wall, said cylindrical wall being coupled to said conductive plate; a plurality of fins disposed radially within said radial waveguide to provide an electrical termination to a selected radial waveguide mode; and feed means coupled to said planar wall to excite the radiation apertures with radial and circumferential currents which provide linearly polarized radiation from each of the apertures.

3. A radiator comprising: a planar conductive plate including radiation apertures disposed along a plurality of concentric circles; a radial waveguide having a planar wall and a cylindrical wall, said cylindrical Wall being coupled to said conductive plate; a plurality of fins disposed radially within said radial waveguide to provide an electrical termination to a selected radial waveguide mode; and two feed means coupled to said planar wall, said feed means being separated by a predetermined distance from one another and from said concentric circles to excite the apertures with energy providing like polarized radiation from each.

4. A radiator comprising: a planar conductive plate including radiation apertures disposed along a plurality of concentric aperture circles about a first axis; a radial waveguide having a planar Wall and a cylindrical wall, said cylindrical wall being coupled to said conductive plate; short circuit means disposed within said radial waveguide about a second axis to provide a short circuit to selected currents in said conductive plate; and first and second electro-magnetic wave energy feed means coupled to said planar wall, said first feed means being disposed symmetrically about said second axis and said second feed means being disposed about a third axis to provide concurrent radial and circumferential currents in said conductive plate which excite the radiation apertures in like fashion, said first, second and third axes being parallel and separated by predetermined distances.

5. A radiator comprising: a planar conductive plate including radiation apertures; first exciting means coupled to said conductive plate for exciting standing wave citcumferential bands of circumferential currents within said conductive plate about a first point; and second exciting means coupled to said conductive plate for exciting standing wave circumferential bands of radial currents within said conductive plate from a second point, said first point and said second point being separated by a distance equal to one-quarter of the wavelength of said current bands, said radiating apertures included with in said conductive plate being disposed along a plurality of concentric circles about a third point, the radius of said circles differing an integer number of wavelengths '7 of said current bands, said first, second and third points defining the vertices of an isosceles triangle wherein the vertex defined by said second point subtends a right angle.

6..A radiator comprising: planar conductive plate including radiation apertures; a first exciting means coupled to said conductive plate and disposed for exciting a current corresponding to the H -mode of the radial Waveguide within said conductive plate symmetric with respect to a first axis; a second exciting means coupled to said conductive plate and disposed for exciting a current corresponding to the E -mode of the radial waveguide within said conductive plate symmetric with respect to a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy exciting said conductive plate; and said radiating apertures contained Within said circular conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles differing by an integer number of Working wavelengths of wave energy exciting said conductive plate, said first, second, and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

7. A radiator comprising: a radial waveguide means having a first axis and including a planar conductive plate, said plate containing radiation apertures; a first exciting means coupled to said waveguide means and disposed for exciting the H -mode of the radial Waveguide within said waveguide means symmetric with respect to said first axis; a first short circuit member concentric with said first axis; a second exciting means coupled to said waveguide means and disposed for exciting the E -mode of the radial waveguide within said waveguide means symmetric with respect to a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy within said waveguide means; and a second short circuit member within said waveguide means and concentric with said second axis, said radiating apertures contained within said conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles differing by one wavelength of the working wavelength of wave energy within said waveguide means, said first, second, and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

8. A radial waveguide antenna for radiating a linearly polarized wave energy pencil beam comprising: a radial waveguide concentric with a first axis and including a .planar circular conductive plate, said plate containing radiation apertures; a first exciting means coupled to said radial waveguide and disposed for exciting the H -mode of the radial waveguide within said radial waveguide symmetric with respect to said first axis; a first short circuit member concentric with said first axis radially terminating said radial waveguide; a second exciting means coupled to said radial waveguide and disposed for exciting the E -mode of the radial waveguide within said radial waveguide symmetric with respect to a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy within said radial waveguide; and a second short circuit member within said radial waveguide and concentric with said second axis, said radiating apertures contained within said circular conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles differing by one wavelength of the working wavelength of wave energy within said radial Waveguide, said first, second, and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said sec- ,ond axis subtends a right angle.

9. A radial waveguide antenna for radiating a linearly polarized wave energy pencil beam comprising: a radial waveguide concentric with a first axis and including a circular conductive plate, said plate having radiation aperture's; 'a circular waveguide coupled to said radial waveguide and disposed coaxially with said first axis; a short circuit member concentric with said first axis coupled to said radial waveguide and a coaxial waveguide coupled to said radial waveguide and concentric with a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy within said radial waveguide, said radiating apertures included within said plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles difiering by one wavelength of the Working wavelength of wave energy within said radial waveguide, said first, second and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

10. A radial waveguide antenna for radiating a linearly polarized wave energy pencil beam comprising: a radial waveguide concentric with a first axis and including a planar circular conductive plate, said plate containing radiation apertures; a circular waveguide coupled to said radial waveguide and disposed coaxially with said first axis; a first short circuit member concentric with said first axis coupled to said radial waveguide; a coaxial waveguide coupled to said radial waveguide and concentn'c with a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy within said radial waveguide; a second short circuit member within said radial waveguide and concentric with said second axis, said radiating apertures contained within said circular conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles difiering by one wavelength of the working wavelength of Wave energy within said radial waveguide, said first, second, and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

11. A radial waveguide antenna for radiating a linearly polarized wave energy pencil beam comprising: a radial waveguide concentric with a first axis and including a planar circular conductive plate, said plate containing radiation apertures; a circular waveguide coupled to said radial waveguide and disposed coaxially with said first axis for exciting the H -mode of the radial waveguide within said radial waveguide; a first short circuit member'concentric with said first axis radially terminating said radial waveguide; a coaxial waveguide coupled to said radial waveguide and concentric with a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the working wavelength of wave energy within said radial waveguide, said coaxial waveguide exciting the E -mode of the radial waveguide within said radial waveguide; and a second short circuit member within said radial waveguide, and concentric with said second axis, said radiating apertures contained within said circular conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles diflering by one wavelength of the working wavelength of wave energy within said radial waveguide, said first, second and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

12. A radial waveguide antenna for radiating a linearly polarized wave energy pencil beam comprising: a radial Waveguide concentric with a first axis and including a planar circular conductive plate, said plate containing radiation apertures; a circular waveguide dimensioned for propagation of the TE -mode of the circular waveguide coupled to said radial waveguide and disposed coaxially with said first axis for exciting the H -mode of the radial waveguide within said radial waveguide; a first short circuit member concentric with said first axis radially terminating said radial waveguide, said first short circuit member providing an electrical short to the H -mods of the radial waveguide; a coaxial waveguide dimensioned for propagation of the TEM-mode of the coaxial waveguide coupled to said radial waveguide and concentric with a second axis, said first and second axes being parallel and separated by a distance equal to one-quarter of the Working wavelength of wave energy within said radial waveguide, said coaxial waveguide exciting the E -mode of the radial waveguide within said radial waveguide; and a second short circuit member within said radial Waveguide and concentric with said second axis, said second short circuit member providing an electrical short to the E -mode of the radial waveguide and permitting unhindered propagation of the H -mode of the radial waveguide, said radiating apertures contained within said circular conductive plate being disposed along a plurality of concentric circles about a third axis parallel to said second axis, the radius of said circles diifering by one wavelength of the working wavelength of wave energy within said radial waveguide, said first, second, and third axes defining the vertices of an isosceles triangle wherein the vertex defined by said second axis subtends a right angle.

13. A radiator comprising: a planar conductive plate including radiation apertures; a radial waveguide coupled to and substantially coextensive with said planar conductive plate; and means coupled to said radial Waveguide for feeding a pair of wave energy modes thereto to establish a pair of standing waves, the standing waves being related to the position of the radiation apertures so as to produce like linearly polarized radiation therefrom.

Lindenblad May 7, 1957 Bickmore June 10, 1958 

